Here's what Jeremy himself posted on the discussion boards:

As Jake.Hazelip and JamieZ indicated (as well as some others),
traditional geocaching itself will never be a pay to play service.

There were some realistic reasons to decide to do a subscription
service. First, the cost of servers are getting to the point that out of
pocket costs and shirt sales no longer support it. As the FAQ said on
the subscription page, we're up to 4 machines in production, not
counting these forums (hosted by Killerlink), as well as a new
mailserver and 2 dev servers. We're even maxing out our bandwidth, so
we'll have to pay for more, which isn't pretty either.

To offset these costs, a subscription service over using annoying mega
size ads and popunder ads seemed like a good option.

The other issue, that I haven't addressed on the subscription page, is
that some folks have become targets by anonymous troublemakers that like
to destroy individual's caches. Subscription only caches will allow you
to audit your cache to see who looked at your cache page and when. In
addition, a certain amount of anonymity goes away when you subscribe, so
troublemakers will either not bother subscribing or be scared to
subscribe since they are no longer anonymous.

In addition to these two reasons, I will also be creating new
geolocational entertainment services in the future that will be handled
by the subscription service. Becoming a charter member now means you'll
be locked in at the same rate throughout, which is always a good thing.

Ultimately, we'd like to do this full time, and we need to eat. So it
has to come from somewhere. And believe me, this has become more than a
1 person job.